The delivery of advertisements on the Internet involves several parties: website content publishers, advertisers, advertisement networks, service providers and end users. A publisher provides web pages of content to the end users and space alongside or within the content for advertisements. The publisher may employ an advertisement network, e.g., Google AdSense, to allow advertisers to bid for placement of advertisements in the ad space. Alternatively, the publisher may arrange directly with advertisers to fill the ad space. In either case, the content with advertisements positioned in the ad space is delivered to the end user over a service provider network. If the end user selects an advertisement, the advertiser pays the publisher or the advertising network for the so-called “click-through” or “click” by the end user.
A rampant problem today is so-called “click fraud,” whereby fraudulent click-throughs are generated, costing advertisers large amounts of money. In one type of click fraud, an unscrupulous competitor may generate a large number of click-throughs in order to deplete the advertiser's advertising budget. In another type of click fraud, a rogue publisher sets up a website that contains only advertisements, and generates a large number of click-throughs at the website in order to obtain the advertising revenues. According to one estimate, click fraud constitutes as much as 20% of all Internet traffic.